The Collaborative International Dictionary of English v.0.48:

Sambo \Sam"bo\, n. [Sp. zambo bandy-legged, the child of a negro
   and an Indian; prob. of African origin.]
   1. A negro; sometimes, the offspring of a black person and a
      mulatto; -- formerly used colloquially or with humorous
      intent, but now considered offensive or racist by
      African-Americans. [Derogatory and offensive]

   Note: A children's book named Little Black Sambo was at one
         time popular in elementary schools, but the book has
         been removed from reading lists in many American
         schools due to the development of a reduced acceptance
         of patronizing depictions of negros, as well as the
         negative associations of the word itself.
         [Webster 1913 Suppl. +PJC]

   2. In Central America, an Indian and negro half-breed, or
      mixed blood.
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Sambo \Sam"bo\, n. [Sp. zambo, sambo.]
   A colloquial or humorous appellation for a negro; sometimes,
   the offspring of a black person and a mulatto; a zambo.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "sambo":
   Cape Colored, Eurasian, cattalo, citrange, cross, crossbreed,
   griffe, half blood, half-bred, half-breed, half-caste, high yellow,
   hinny, hybrid, ladino, liger, mestee, mestiza, mestizo, metis,
   metisse, mixblood, mixed-blood, mongrel, mulatto, mule, mustee,
   octoroon, plumcot, quadroon, quintroon, tangelo, tigon, zebrass,
   zebrule